Abstract Premature birth remains a significant issue in the United States, with 11. 6% of all births occurring at a gestational age under 37 weeks in 2013, with over 1. 5% of these deliveries at the high-risk period under 32 weeks gestation. Current literature shows that outcomes (mortality and serious morbidity) for high-risk neonates, especially extremely premature infants, are improved if infants are delivered at a hospital with a high-volume, tertiary-level neonatal intensive care unit (NICU). While the evidence suggests that neonatal care should be more ?regionalized?, there is no information on how this process should occur. There are no large-scale studies with appropriate methods that have examine how outcomes change within hospitals in response to changes in NICU volume and/or NICU level of care. Before any policy recommendation to regionalize care can be enacted, a better understanding of these effects is needed. In its last revision of standards for NICU levels, the American Academy of Pediatrics determined that while there is some evidence that patient volume should be included in the NICU guidelines, that there wasn't sufficient evidence to include volume in the standards for NICU levels, and that the critical volume thresholds needed to be identified. The implications of better regionalization of neonatal care are profound; the perinatal mortality rate in Portugal decreased by 60% in the 1990s following a mandated regionalization of all perinatal care. Estimates derived from cross-sectional US studies indicate 20-40% reductions in perinatal mortality may be attainable. To provide a better understanding of the potential for better regionalization of perinatal care to actually reduce perinatal mortality, the proposed study will address the following specific aims: Aim 1: Examine within hospital changes in in NICU volume and NICU level of care to determine how these changes affect outcomes, such as mortality and serious common complications of preterm birth. Aim 2: Is there an interaction between initial outcomes (quality of care) and how changes in volume and level of care affect outcomes? We will use linked vital statistics-patient discharge data (VS-PDD) from California, Missouri, and Pennsylvania from 1991-2014, supplemented by other datasets. This range of data allows sufficient variation in NICU levels and volumes to test the study hypotheses. Statistically, we will exploit the panel nature of the data (repeated observations of each NICU over time) to control for unobserved, hospital- specific factors that affect outcomes, and include additional methodologies to account for other potential sources of unobserved confounding. At the end of this study, we will have identified the structures and patient volumes needed to optimize the outcomes of premature infants, allowing for evidence-based policies at the state and national-level to guide the development of perinatal delivery systems.